Bottom bracket threading

[beanzontoast]

I've cleaned and replaced sealed unit bottom brackets before, but have never come across this...

On removing the bb unit from the chain side, I noticed the threading doesn't seem to engage tightly while being withdrawn - there is back and forward 'play' - that is, the bracket will move in and out of the socket after only a few turns of the removing tool while there's loads of thread on the bb unit still technically to be unscrewed. And only on the final few turns of the bb tool when refitting it does it seem to grip the bb socket.

Now, I haven't had any problem with the bb or the cranks - no noticable excess play in them while riding - but I'm wondering if I should get the LBS to examine it. Could it be a case of the bb socket threads needing re-cutting? (Any idea of cost?). Or is it more likely to be the threads on the bb cartridge that are at fault? I suppose it's not likely that a previous owner has installed a bb cartridge of slightly the wrong diameter? The frame is 531 tubing by the way.

Any ideas what I'm looking at here?

 

[MartinC]

The threads on the BB shell are worn or a poor fit - the threads are a loose fit. Size isn't an issue really - currently there are only two standards really English and Italian. English BB's have a left hand thread on one side it's not possible that you're using the wrong sort. If the BB shell threads are worn then getting them recut won't help. It may be that the BB cup is poorly made and very slightly undersized - try another and see if it cures the problem. If it's the shell that's worn the a framebuilder (e.g. Mercian or Argos) can replace it but it will be expensive a require a respray. Or you could just use lots of Loctite. Stronglight and Mavic used to make BB's that would fit in shells where the threads had been stripped but I'm not sure if these are still available.

 

[mickle]

Wot Martin sed.

 

[beanzontoast]

Thanks both - confirms what I thought but wasn't brave enough to write .

I have a mate who has an engineering workshop so I think I'll tell him what you've said and see where he goes with it from there.

 

[Monty Dog]

Yes, worn threads - shouldn't really be a problem as once you've screwed in the BB, the tension is the threads should holds everything firmly in place. I'd only get it seen to if you've actually stripped the thread because the cost of a new BB shell and respray is going to be prohibitive - £150-200. Sadly the Mavic 630 BB is no longer producer and was only available in certain lengths - but there is one on ebay right now! but needs a specific chamfer cut into the BB shell to engage which your engineering mate might be able to do for you. It's a great, strong unit. If you're worried about it loosening, using a lighter grade of Loctite (not threadlock) will stop it coming loose.

 

[beanzontoast]

Monty - Will have to see what he says. Will be a couple of weeks before I get it over to him. As you say, if it's the worst it can be, it could cost an arm and a leg, and might not be worth it ...

 

[Dave5N]

AHem. PTFE tape works.

(just don't tell anyone I did this.)

 

[TheDoctor]

If the threads are stuffed (and it sounds like they're not) might I suggest the YST BB at Byercycles 113mm, 119 mm or 122.5mm for £13.95.
Usual disclaimer.

 

[beanzontoast]

If the threads are stuffed (and it sounds like they're not) might I suggest the YST BB at Byercycles 113mm, 119 mm or 122.5mm for £13.95.
Usual disclaimer.

Understood. That's really interesting, though - thanks.